1. Field of the Invention
The present invention relates to a roller for fixing used in an image fixing apparatus included in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, and an image fixing apparatus using the roller for fixing.
2. Description of the Related Art
Electrophotographic copying machines and electrophotographic printers include an image fixing apparatus for heating and fixing, on a recording material, an unfixed toner image carried by the recording material.
A roller for fixing such as a fixing roller or a pressure roller included in an image fixing apparatus needs a releasing layer for preventing adhesion of toner. A method of providing a releasing layer includes a method of covering a surface of a roller for fixing with tube-shaped fluororesin, or a method of coating a surface of a roller for fixing with a liquid containing dispersed fluororesin powder, and then melting the fluororesin by calcining to form a releasing layer of fluororesin.
There is a roller for fixing desirably having a layer configuration including a thermal insulation layer having a high thermal insulation property inside, and a thermal storage layer having a high volume heat capacity near a surface. An example thereof is a roller for fixing suitable for a fixing apparatus in which a heat source is not provided inside the roller for fixing, but a surface of the roller for fixing is heated from outside the roller for fixing to store heat near the surface, and the heat is provided to a toner image on a recording material in a fixing nip portion. Another example is a pressure roller suitable for a film heating type fixing apparatus, including an endless film, a heater that comes into contact with an inner surface of the endless film, and a pressure roller that forms a fixing nip portion with the heater via the endless film. A surface of the pressure roller as a roller for fixing is formed as a thermal storage layer (having high thermal conductivity) to allow heat on the surface of the pressure roller to be stored and transferred, and thus a fixing temperature and electric power can be reduced.
When a releasing layer is formed of tube-shaped fluororesin in such a roller for fixing, the tube-shaped fluororesin has a thickness of about 30 μm, and this prevents heat supplied from outside the roller for fixing from being transferred to the thermal storage layer. Meanwhile, with a method of coating with a liquid containing dispersed fluororesin powder, a thickness of a releasing layer can be reduced. However, a thermal insulation layer significantly expands in a process of calcining the liquid coated on the surface of the roller for fixing, and the expansion affects the liquid containing dispersed fluororesin via the thermal storage layer. When the thermal insulation layer expands, the coated fluororesin powder in the liquid is dissociated before melted, and even if the fluororesin powder is melted by calcining, the fluororesin layer is separated. Then, even if the thermal insulation layer is cooled and shrinks after the calcining, the fluororesin layer remains separated (cracked) or wrinkled. The fluororesin layer separated or wrinkled does not function as a releasing layer.
As a method of forming a thin releasing layer with reduced cracks, a method of coating fluororubber latex on a thermal storage layer is supposed. The fluororubber latex, generally in a dispersion state, is applied to a surface of a roller for fixing, and calcined at a temperature of about 300° C. Components of the fluororubber latex include fluororesin, and the fluororesin seeps out to the surface of the roller for fixing by calcining to form a releasing layer. Thus, the fluororubber latex is separated into a fluororubber layer and a fluororesin layer in the calcining process. The fluororubber latex has rubber elasticity (stretchability), and thus even if a thermal insulation layer thermally expands in the calcining process, a thin fluororesin releasing layer with reduced cracks or wrinkles can be formed. Japanese Patent Application Laid-Open No. H10-115991 discloses a pressure roller including, on a core metal, a thermal insulation layer of sponge, a high thermal conductive silicone rubber layer, and a fluororubber latex layer.
To store heat in a thermal storage layer, thermal conductivity of a fluororubber latex layer needs to be also increased. To increase the thermal conductivity of a fluororubber latex coat, high thermal conductive filler needs to be dispersed therein. To obtain at least an effect of increasing thermal conductivity of the fluororubber latex coat, a volume fraction of the high thermal conductive filler in the fluororubber latex coat needs to be about 5% or more. However, the fluororubber latex layer containing the dispersed high thermal conductive filler eventually has low rubber elasticity. This reduces a function of the fluororubber latex of preventing cracks or wrinkles in a releasing layer, and causes wrinkles to easily occur on the surface during production.